Scrapie is a transmissible neurodegenerative disease that is caused by infectious pathogens called prions. The only macromolecule that has been identified in the prion is a disease-specific isoform of the prion protein (PrPSc), which is encoded by a host gene. Examination of both natural and experimental prion diseases indicates that host genes exert a major influence on disease. Genetic analysis of scrapie in mice provided the first evidence that the prion protein gene might modulate susceptibility to prion diseases. A prion incubation time gene (Prn-i) is tightly linked to the prion protein gene (Prn-p) on mouse chromosome 2. Long incubation time Prn-pb mice encode PrP-B proteins that differ from PrP-A of short incubation time mice by two amino acids. However, transgenic (Tg) mice expressing Prn-pb had shorter incubation times than non-Tg controls, and the nature of the mouse Prn-i gene is unresolved. The hypothesis that Prn- i is distinct from Prn-p will be tested by exploiting Prn-p congenic mouse strains to facilitate genetic and physical mapping of the Prn-p region using yeast artificial chromosomes. Large segments of genomic DNA adjacent to Prn-pb will be recovered in P1 vectors; inserts containing candidate Prn-i loci will be used to construct Tg mice. The hypothesis that Prn-i is a regulator of Prn-p expression will be tested by comparing mRNA and PrP expression in Prn-pa, Prn-pb and Tg (Prn-pb) mice. Tg mice also will be constructed with large Prn-pb containing inserts. The hypothesis that Prn- i and Prn-p are equivalent will be tested by determining whether inappropriate expression of PrP-B in Tg mice masks the effects of an endogenous Prn-i gene, and by targeting Prn-pb mutations through homologous recombination. Preliminary results from embryo aggregation chimeras suggest that Prn-i can prolong incubation time even when not expressed in all cells of the mouse. Examination of the mode of action of Prn-i/Prn-p and the relationship of disease course to pathological lesions will utilize embryo aggregation chimeras. Results from both mice and humans suggest that loci other than Prn-p/Prn-i can dramatically influence prion disease phenotype. Interspecific backcrosses will be used as the first step in analysis of polygenic modulation of prion diseases. Results from this project will improve our understanding of the mechanisms for prion replication and disease susceptibility and shed new light on neurodegenerative disease in general.